Personal identification is demanded in various situations in modern society. As one of methods for accurately performing personal identification, a biometric authentication technique in which individual authentication is performed by using a feature of a biological part of a human body such as a fingerprint, a vein, and a face has been widespread in recent years. For example, biometric authentication is used for determination of propriety of entering and leaving a restricted area to which only a permitted person may enter, log-in determination to a personal computer, and determination of propriety of access to various services such as online transaction.
To perform such biometric authentication, an image of a biological part such as a fingerprint, a vein, and a face is first acquired preliminarily as biological information of a user so as to be stored in a storage medium as registered biological information of the user. Afterward, when personal identification for a use of above-described services is demanded, biological information of the user is acquired again in a similar manner to the registration time. Then, comparison and collation between the biological information (authentication biological information) acquired at this time and the above-described registered biological information which is stored in the storage medium is performed so as to determine the degree of similarity of the both information. When the degree of similarity is higher than a predetermined threshold value, an authentication result authenticating an identical person is obtained.
Here, in photographing, which is performed to acquire biological information, of a biological part with an image sensor such as a camera, there is a case where a physical feature which is used for biometric authentication is not appropriately detected from an image of a biological part in a photographed image due to surface reflection of light on the biological part.
Surface reflection is a phenomenon that light reflects on a surface of an object. When a surface of an object is planar, light regularly reflects due to surface reflection as illustrated in FIG. 1A, and thus an incident angle (an angle formed by a normal line with respect to the surface and a direction of the incident light) θi and a reflection angle (an angle formed by the normal line with respect to the surface and a direction of reflection light) θr have the same angles as each other.
However, almost all objects in nature do not have completely planar surfaces and have slightly uneven surfaces, so that reflection light generated by surface reflection is actually radiated to directions spread to some extent as illustrated in FIG. 1B. The degree of the spread of the reflection light varies depending on the degree of unevenness of an object surface. For example, a human skin has a surface having large unevenness, so that reflection light generated by surface reflection on the skin is radiated to a relatively wide range. However, in such surface reflection, the intensity of reflection light in a direction of a reflection angle θr which is approximately equal to an incident angle θi is the strongest and the intensity gradually lowers as the direction varies from the direction of the reflection angle θr.
As described above, though reflection light generated by surface reflection slightly spreads, the reflection light basically has high angle dependency. Here, reflection light on an object includes reflection light generated by diffuse reflection. Diffuse reflection is a phenomenon that incident light penetrates the inside of an object, repeats diffusion, and then, is radiated from a surface of the object, as illustrated in FIG. 2.
Reflection light generated by diffuse reflection has low angle dependency with respect to an incident angle of incident light and spreads evenly in all azimuths, so that the intensity of the reflection light is approximately even independently of directions. In a precise sense, the intensity of reflection light generated by diffuse reflection in an immoderate direction (for example, a direction in an angle almost parallel to a surface of an object) lowers. However, such direction is excluded from the consideration here.
In the biometric authentication technique, an image formed by reflection light which is generated by diffuse reflection on a biological part is mainly used. For example, in vein authentication using a vein of a palm, an image based on reflection light which is generated from a near-infrared ray which is radiated to a skin and diffuse-reflects at a vein under the skin to return is taken to acquire a vein pattern under the skin and the vein pattern is extracted from this image so as to be used for biometric authentication. FIG. 3A illustrates an example of a case where extraction of a vein pattern from an image may be appropriately performed.
On the other hand, if a component based on surface reflection on a skin surface is included in the reflection light, part of an image in a photographed image may be too bright to cover over a vein pattern under the skin or components of a corrugated surface may be superposed on the vein pattern under the skin, as examples of images indicated in FIG. 3B. From such image, it is hard to appropriately obtain the vein pattern.
Such effect caused by surface reflection of light is similarly observed in a case of acquiring biological information mainly existing on a biological surface, such as a fingerprint and a face. If a bright spot is generated on a part of an image of a photographed image due to surface reflection of light, it is difficult to appropriately obtain biological information.
Several techniques have been disclosed to remove such effect of surface reflection components. For example, Japanese Laid-open Patent Publication No. 2003-331270 discloses a technique in which polarization filters are disposed in front of illumination and a camera so as to optically remove surface reflection components. In this technique, surface reflection components are removed with a polarization filter through which only light in a specific polarization direction passes by using such a property that a polarization direction of incident light of surface reflection light does not change but a polarization direction of diffuse reflection light changes. That is, in this technique, polarization filters, which have polarization properties orthogonal to each other, are respectively disposed in front of illumination and a camera so as to remove light having polarization components same as polarization components of light with which a biological object is irradiated before the light reaches the camera, thus removing only surface reflection components.
Further, for example, Japanese Laid-open Patent Publication No. 10-162146 discloses a technique in which a plurality of images, which are obtained by photographing every time lighting patterns of a plurality of illuminations which are disposed to surround a camera are changed, are composited after removing parts of over-exposure caused by surface reflection, so as to obtain one image. The intensity of a surface reflection component largely depends on a positional relationship among illumination, a camera, and a photographing object, so that parts having strong surface reflection components are different among a plurality of images obtained by photographing every time an irradiation angle of light is changed by changing a position of illumination. Therefore, in this technique, the images after removing parts of over-exposure caused by surface reflection are composited so as to obtain one image having no surface reflection.
As other related art technique, Japanese Laid-open Patent Publication No. 2004-030564 discloses a technique in which authentication processing is executed to each of a plurality of images which have different projected positions of external light, so as to obtain a final authentication result based on a plurality of authentication results which are obtained by the authentication processing.